Television system



Feb. 2, 1943. T. GOTTlE- IR 2,309,754

TELEVIS ION SYSTEM Filed Sept. 23, 1941 L OUTPUT 3Q BIASES Tl/BL;

[ -n'j BEYOND curor ME YIN G PULSE-5 HABAAAA Wvvvvvv WHITE BLACK 1 lFREQ, OF ERYSTAL osc MAXIMUM OSIRD' FREQ. 0WA no 0 v BLACK 5105 T [HESnnenfor FREQUENCY THOMAS EARMER Gonm Patented Feb. 2, 194a ThomasLarimer signor to Radio poration of Delaware Application September 23,1941, Serial No. 411,949 Claims. (01. 178-7.!)

My invention relates to the art of frequency modulation, particularly asapplied to television or other picture transmission systems.

In television systems employing amplitude modulation, it has been theusual practice to so modulate a carrier wave that the synchronizingpulses of the modulating signal modulate the carrier wave either .to apredetermined maximum n amplitude or to a predetermined minimumamplitude such as zero carrier, the two ways of modulating beingreferred to as negative and positive modulation, respectively. Themodulating signal referred to is a mixture of synchronizing pulses andpicture signal, the said pulses being in the direction of ffblack in thepicture.

An object of the present invention is to provide an improved frequencymodulation system for a television or picture transmitter which re tainsthe above-mentioned feature of having the synchronizing pulses always goto a predetermined modulation' level. This predetermined level in afrequency modulation system is, of course, a predetermined frequency.

A further object of my invention is to'provide a method of and means forholding the mean frequency of a frequency-modulated carrier wavesubstantially constant.

In one preferred embodiment of the invention,

the peaks of the synchronizing pulses are held at the desiredpredetermined level or frequency by applying the output of a stableoscillator, such as a crystal osci lator, to the carrier wave oscillaterduring the occurrence of each synchronizing pulse. The crystaloscillator frequency is the frequency to which the synchronizing :pulsesin the modulating signal are supposed to deviate the carrier wave. Uponthe occurrence of each synchronizing pulse, the crystal oscillatorwillpull the carrier wave oscillator to the said frequencyif it'has notalready been modulated to the exact desired frequency.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which Figure 1 is acircuit diagram of a portion oi a television transmitter embodying myinvention, and

Figure 2 is a diagram which is referred to in explaining the operationof the invention.

Referring to Figure 1, there is shown a television transmittercomprising a carrier wave oscillator Hi which may he of a well-knowntype liavingmi tuned circuit H and a condenser l2 connecting one end ofthe tuned circuit to the grid of the oscillator tube I3.

l at which the diode 26 passes current. Battery Gottier, Westmont, N.3., as- Corporati on of America, a cor- The output of oscillator i0 isfrequency-modulated by the video input or television signal which isamplified by a video amplifier I 6 and applied through a blockingcondenser I1 and an inductance coil IB-to the grid of a reactance tubei9 The function of the reactance I 9 is to vary the effective reactanceacross the oscillatortuned circuit ii in accordance with the voltageamplitude variations on the grid of the reactance tube. Thus, afrequency-modulated carrier wave is obtained. In the particular exampleshown, the tube is is connected across the tuned circuit I! by aconnection from the plate of tube l9 to the plate of oscillator tube Itand by a connec-- tion from the cathode of tube E9 .to ground. Aresistor 29 connected between the grid and plate of tube it) through ablocking condenser 22 functions together with the grid-cathode capacityindicated at 23 to provide the necessary out-of-' phase componentforlthe reactance tube operation.

A diode it is provided to reinsert the direct current component of thevideo signal. The D. C. reinserticn circuit operation is well known andmay be explained briefly as follows: The synchronizing pulses are ofpositive polarity at .the plate of diode 26 and will cause a flow ofdiode current whenever they drive the diode. plate positive with respectto its cathode. Thisputs a charge on coupling condenser l1. At the endof a pulse, the condenser ll loses a small part of its charge through a.resistor 21 whereby the D. C. voltage across condenser i 1 may alwayshave a value determined by the peaks of the synchronizing pulses asmeasured from the A. 0. axis of the signal.

The biasing battery 28 sets the voltage level 28 also applies a negativebias to the control grid of tube It so that the synchronizing pulsesnever drive .the grid of tube is positive with respect to'its cathode.

1 Inductance coil It is provided to separate. the electrode capacitiesof tubes is and 26 for the purpose of holding up the high frequencyresponse.

For holding the peaks of synchronizing pulses at a flxedfrequency, astable oscillator, such as a crystal oscillator to, is provided whichproduces a voltage at a frequency equal .to the said fixed'fre'quencyrepresenting peaks of synchronizing pulses.

In Figure 2, the frequency of oscillator 30 is indicated by the verticaldotted line. This also is the frequency at which the peaks of thesynchronizing pulses are to be held. The synchronizing pulses and thepicture signal which comprise, the modulating signal are indicated at 3|and 32, respectively.

The output of oscillator 30 is fed to the grid of an amplifier which,between synchronizing pulses, is biased beyond cut-of! by a biasingbattery-31m The plate circuit of amplifier 36 is connected to a point onthe tuned circuit H so that, during a synchronizing pulse, it presentsto the carrier wave oscillator ill a reactance that is either inductiveor capacitive, depending upon the instantaneous phase difference of thetwo oscillator outputs. Plate voltage is supplied to amplifier 38through a radio frequency choke coil 38.

The pulses which key the amplifier 36 to make it pass signal during theoccurrence of each synchronizing pulse in the modulating signal areindicated at 4|. They may be obtained from any suitable source indicatedat 42 which produces pulses occurring simultaneously with thesynchronizing pulses. The source 42 may be, and preferably is, thesynchronizing pulse generator itself.

From the foregoing description, it will be apparent that, during eachkeying pulse 4|, the crystal controlled oscillator feeds signal at afixed frequency to the tank circuit I I of the carrier wave oscillatorl0 whereby its frequency of oscillation during these periods is shiftedto said fixed frequency if it happens to be above or below it as aresult of frequency drift of oscillator I0, for example.

It will be understood that my invention holds the mean frequency of theoscillator l0 substantially constant in addition to making the oscillator output go to a predetermined frequency during each synchronizingpulse. The reason for this is that the synchronizing pulses, especiallythose occurring at line frequency, have such a short time intervalbetween them that a well designed oscillator will drift in frequency anegligible amount between pulses.

Because of the above-described action, the invention may be applied to asound transmitter for holding the mean frequency constant. In

such a transmitter, special keying pulses must be supplied. Theypreferably are made to occur at a superaudible rate to avoid thenecessity of filtering themfrom the sound output.

I claim as my invention:

1. A radio transmitter comprising a carrier wave oscillator, means forfrequency modulating said oscillator in accordance with a modulatingsignal having a maximum amplitude represented by a maximum deviation ofsaid oscillator frequency to a predetermined frequency, a stableoscillator which oscillates at said predetermined frequency, means forsupplying recurring control pulses, and means for coupling said stableoscillator to said carrier wave oscillator during the occurrence of saidcontrol pulses only.

2. A picture transmitter comprising a carrier wave oscillator, means forfrequency modulating said oscillator in accordance with a signal com-"prising picture signals and recurring syncoupling said stableoscillator to said carrier.

wave oscillator during the occurrence of said synchronizing pulses only.

3. A picture transmitter comprising a carrier wave oscillator, meansincluding a reactance tube for frequency molulating said oscillator inaccordance with a signal comprising picture,

signals and recurring synchronizing pulses whereby the peaks by a,certain maximum deviation of said oscillator frequency to apredetermined frequency, and means for causing the peaks of said pulsesalways to swing the output of said oscillator to said predeterminedfrequency, said last means comprising a stable oscillator whichoscillates at said predetermined frequency, and means for coupling saidstable oscillator to said carrier wave oscillator during and only duringeach of said synchronizing pulses.

4. A picture transmitter comprising a carrier wave oscillator, means forfrequency modulating said oscillator in accordance with a signalcomprising picture signals and recurring synchronizing pulses which goto a fixed voltage level whereby the peaks of said pulses arerepresented by a certain maximum deviation of said oscillator frequencyto a predetermined frequency, and means for causing the peaks of saidpulses always to swing the output of said oscillator to saidpredetermined frequency regardless of the tendency of said carrier waveoscillator to drift in frequency, said last means comprising apiezo-electric crystal oscillator, and means for feeding its output intosaid carrier wave oscillator during and only during the occurrence of,each of said synchronizing pulses.

5. A frequency modulation transmitter comprising a carrier waveoscillator, means for producing a modulating signal containingperiodically recurring pulses which go to a predetermined voltage level,means for varying the frequency of said oscillator in accordance withthe instantaneous amplitude of said modulating signal, and means forlimiting said frequency variation so that it does not exceed a certainfrequency limit for the lower frequency components of said pulses, saidlast means comprising a stable oscillator which oscillates at saidfrequency limit and which is coupled to said carrier wave oscillatorthrough an amplifier tube; and means for making said amplifier tubeeffective to transfer said stable oscillator output tosaid carrier waveoscillator during and only of said pulses are represented-

